The use of sealed capsules is well known in circumstances where the force of the fluid under pressure is employed to dispense a substance such as discharging draught beer from a beer dispenser or expelling soda water from a soda siphon.
It is also known to employ sealed capsules containing helium at high pressures, in the order of 30-40 bar, in medical devices using the energy of the pressurized helium to drive a therapeutic agent through the skin of a patient.
In PCT published application W094/24263 there is described a needleless syringe, which includes a metal capsule containing helium gas at high pressure which is used to force particles of a therapeutic agent through the skin of a patient in a substantially painless manner. The capsule is detachable from the remainder of the syringe and once used, either a new charge of gas can be placed in the capsule or more favorably, the capsule can be discarded and a new capsule charged with gas can be attached to the remainder of the syringe.
In the circumstance where the gas capsule is a throw away item, it is important that it can be manufactured simply and cheaply. In medical applications, helium gas is a favored fluid since it is very light which makes it suitable for use as a propellant for therapeutic agents in that when it impinges against the skin of a patient it will bounce off into the atmosphere and not pass through. However, helium can be disadvantageous in that, because it is light, it is difficult to contain since it will leak through the most minuscule fault in a container.
Furthermore, in medical applications it is important that the helium gas can be released from the gas capsule with the minimum of force by the user, for example, finger force.
UK Patent 253744 describes a metal capsule for gases or liquids under pressure which can be used to inflate a flexible container. The capsule comprises a cylindrical hollow body having a neck portion formed with an internal thread. A closure member has a corresponding external thread and is screwed into the neck portion.
The closure member has an outwardly projecting stem which is hollowed so that it may easily be broken. The stem has an external thread for connection to a handle forming part of the flexible container. When it is required to liberate the gases or liquids in the capsule, the hollow body is moved laterally relative to the handle and the stem connected thereto such that the stem is ruptured thereby releasing the gas from the interior of the hollow body.
Such a capsule, however, is not effective for containing a very light gas such as helium for a reasonable length of time at pressures in the order of 60 to 80 bar since the helium molecules will leak through the helical path between the co-operating internal thread of the neck portion and external thread on the closure member. Furthermore, in order to break the stem a considerable laterally force needs to be applied, that is, a force greater than finger pressure having no mechanical advantage. A further disadvantage is that the capsule is expensive to manufacture in that screw threads have to be turned on both the neck portion and the closure member.
In accordance with the present invention, a gas delivery system is provided which avoids the disadvantages of the prior art systems. The gas delivery system of the invention contains a capsule for fluid under high pressure which can be manufactured simply and cheaply and which is substantially leakproof even when the fluid under pressure is helium. the capsules of the invention are further advantageous in that they can be opened with finger pressure having no or relatively very little mechanical advantage without causing the gas delivery device of which it forms a part, to move when held in one hand.